It is well known that pain can be alleviated by electrical pulses applied to the surface of the body or to electrodes implanted within the body. Initially, this electrical stimulation was applied in such a manner that the energy was only sufficient to stimulate sensory nerves. Efforts were made to avoid stimulating muscle nerves which produce fasciculations. Subsequently it was found that electrical stimulation at sufficiently high levels to elicit muscle contractions, which are an indication the deep afferent sensory pathways are activated, results in greatly improved long term analgesia. Pain patients are in general unable to tolerate the unpleasant sensations which accompany high intensity stimulation.
To avoid this it has been proposed to stimulate deep sensory nerves which short trains to monophasic pulses, the pulse trains being at a low frequency. By this procedure, the current necessary to stimulate deep sensory nerves can be reduced by one-third to one-half. Conventional transcutaneous electric nerve stimulation usually consists of a continuous train of pulses with three variable parameters. The rate may vary between 1 to 100 pulses per second, output between 0 to 70 milliampheres peak-to-peak, and pulse width between 0 to 400 micro-seconds. High rate transcutaneous electrical nerve stimulation usually refers to rates greater than 50 pulses per second. At these higher rates, if the interval is increased to a level which produces muscle contractions and attended deep sensory pathway activation, few patients can tolerate the resulting sensations. By interrupting the pulse train periodically at a low rate, that is, by cycling the stimulation on and off, the sensations can be reduced to a degree, but generally patients cannot tolerate even interrupted pulses at the levels necessary for deep afferent nerve stimulation.